My rig is I3-2100 3.1Ghz, HD7770 vapor-x, 4GB RAM and running on 1366x768 res monitor
When I set MESH quality to HIGH, fps drop heavily when changing view in all DLC map (B2K, CQ,AK and also AM )
The vanilla map pack is OK.
I have some budget to upgrade. I wonder I should spend it on the new GPU 7850 + PSU 500W or new CPU i5 2500k
All I need is running the game @high mesh quality for long range and air spotting

Do what the other are saying, upgrade your CPU, and add some more ram. If you can find some AMD quadcore it will still be better. For that resolution the 7770 is more than enough, I've played on high with a 5770 at 1360x768

It doesn't. Also depends on the mobo's chipset if he can adjust the BCLK. Anyway he say 100% usage CPU and RAM and 70-80% on the GPU. BF3 multiplayer is hard on the CPU but before you jump on expenses (an unlocked i5) you should really try to put more RAM (it's cheap!) and see if it's better.

According to Techspot's CPU scaling test the Core i3 2120 @3.3 GHz (Your Core i3 2100 is 3.1 GHz) does okay in Battlefield 3, even in 1920x1200 resolution; it's on par with the Core i5 750 @2.67 Ghz, just a little slower than the Core i5 2500K with stock speed and that's with high quality settings for the graphics in Battlefield 3.

According to Techspot's CPU scaling test the Core i3 2120 @3.3 GHz (Your Core i3 2100 is 3.1 GHz) does okay in Battlefield 3, even in 1920x1200 resolution; it's on par with the Core i5 750 @2.67 Ghz, just a little slower than the Core i5 2500K with stock speed and that's with high quality settings for the graphics in Battlefield 3.

Click to expand...

That test and the majority if not all reviews done on BF3 mean absolutely nothing when you engage on a 64 player large map in multiplayer. But I do agree that a Sandy Bridge i3 with hyperthreading should keep up to a point.

My rig is I3-2100 3.1Ghz, HD7770 vapor-x, 4GB RAM and running on 1366x768 res monitor
When I set MESH quality to HIGH, fps drop heavily when changing view in all DLC map (B2K, CQ,AK and also AM )
The vanilla map pack is OK.
I have some budget to upgrade. I wonder I should spend it on the new GPU 7850 + PSU 500W or new CPU i5 2500k
All I need is running the game @high mesh quality for long range and air spotting

Click to expand...

You don't need a CPU upgrade. Heck I can max it out on an Athlon II X4!

A quad core would help but your main bottleneck is the GPU and the RAM. If you are still unhappy then look into a CPU, but I doubt you'll need it.

Just drop in a 7850 and up the ram to 8GB and you'll be able to max it out easy. You are only running 1366x768 so it doesnt need anything special.

Edit:

On second thoughts. You are only running 1366x768, I think your 7770 is enough. I would go for the RAM upgrade and take it from there.

You aren't pushing high resolutions. Unless you plan to get a better monitor, throw out the advice on GPU upgrades. Heck don't even think about Crossfire. Most SLI/Crossfire setups aren't worth the money till you get into 1080p resolutions.

Your looking at RAM and CPU being the most benefit right now.

Now if you want to push Ultra textures and such, you might want something with 2GB VRAM.

Just upgrade a Corsair Vengeance Blue 8 GB (2X4 GB)1600, the fps doesn't change much but some low fps screen (35-40) get smoother, and the system monitor record no use of > 4096 MB just only 3100MB were used when game playing,don't know why